Opening an Icy Eye on the Neutrino Sky

A field of sensors buried deep in Antarctica has produced a novel view of the sky: a map that shows not light but neutrinos, ethereal particles emitted by star-munching black holes, supernova explosions, and other violent phenomena. Neutrinos have no charge and almost no mass, so they zip unflinchingly through space and matter. That quality makes them ideal for probing the most inaccessible corners of the universe but also makes them almost impossible to detect.

A network of sensors, lowered into deep drill holes (left), detects neutrinos from the Northern Hemisphere that penetrate Earth and exit through Antarctica. The preliminary map of neutrino events (right), plotted on an oval projection of the sky, may show both atmospheric and cosmic sources.Photograph and graphic courtesy of the Amanda Project.

To find the astronomical neutrinos, Francis Halzen of the University of Wisconsin at Madison and an international team of colleagues built AMANDA II, an array of 677 bowling-ball-size light detectors strung along 19 cables and sunk nearly half a mile down into the polar ice. Several times a day, a high-energy neutrino from the northern sky burrows through Earth and crashes into a water molecule near one of the sensors. The resulting subatomic chain reaction produces brief trails of blue light, called Cerenkov radiation, zigging through the ice. Those signals are picked up and amplified by the detectors. The path of the radiation indicates the original course of the incoming neutrino.

During AMANDA II's first year of operation, it picked up 1,000 neutrino collisions. Halzen and company processed the data into the first-ever neutrino sky map. The researchers are now combing through a second and third year of data to track down the brightest neutrino emitters. Some may be the long-sought sources of the astoundingly energetic particles called cosmic rays that rain down on Earth from parts unknown. "We have no clue where they come from because their paths are bent and scrambled by Earth's magnetic field," Halzen says. Neutrinos, however, would fly on an arrow-straight path from their point of originperhaps places where hot gas falls into supermassive black holes. "So one exciting possibility is that the neutrino sky map will reveal the sources of cosmic rays as well."